PROJECT SUMMARY. Role of XPD in DNA Damage Response Pathway Choice The DNA damage response has evolved to protect the integrity of the human genome against the potentially devastating effects of endogenous and exogenous genotoxins. A successful DNA damage response occurs due to well-orchestrated crosstalk between pathways within the network. For instance an intricate balance between DNA repair and apoptosis minimizes the likelihood of genomic instability, which can lead to mutagenesis and ultimately to carcinogenesis. Although the regulatory mechanism and signaling pathways controlling DNA repair and apoptosis are well characterized, the driving forces responsible for making the ultimate choice between DNA repair and cell survival or apoptotic cell death in response to genotoxic stress remain unclear. The goal of this project is to understand the mechanisms involved in triggering the appropriate reaction at the intersection of the nucleotide excision repair (NER) and apoptotic pathways. The NER pathway occupies an important position in the recognition and repair of a wide array of helix- distorting lesions, including formation of triplex or H-DNA structures. Recent work from our group has discovered that the NER protein, XPD occupies a central role in triggering apoptosis in response to helix- distorting DNA damage. XPD forms a complex with ?H2AX and its presence is required for the phosphorylation of the H2AX tyrosine142 residue, which stimulates the signaling pathway to recruit pro-apoptotic factors to the damage site. Using triplex formation as a model DNA damage lesion, we will elucidate the mechanisms utilized to maintain effective crosstalk between NER and apoptotic pathways in the context of the XPD protein, as proposed in Aim 1. We will also determine how tyrosine phosphorylation of XPD functions as a signaling mechanism to stimulate XPD-dependent apoptosis. In Aim 2, we will differentiate XPD's functionality in NER from its role in apoptosis. These studies will include evaluating the impact of XPD mutations associated with human genetic diseases and cancer on downstream signaling. The proposed work is highly significant because imbalance between these two pathways can eventually contribute to the etiology of human diseases, such as cancer.